Semiconductor device and method of manufacturing the same

ABSTRACT

A semiconductor device has a channel layer formed above a substrate, a barrier layer formed over the channel layer and having a band gap larger than that of the channel layer, a trench passing through the barrier layer as far as a midway of the channel layer, and a gate electrode disposed byway of a gate insulation film in the inside of the trench. Then, the end of the bottom of the trench is in a rounded shape and the gate insulation film in contact with the end of the bottom of the trench is in a rounded shape. By providing the end of the bottom of the trench with a roundness as described above, a thickness of the gate insulation film situated between the end of the bottom of the gate electrode and the end of the bottom of the trench can be decreased. Thus, the channel is formed also at the end of the bottom of the trench to reduce the resistance of the channel.

CROSS-REFERENCE TO RELATED APPLICATIONS

The disclosure of Japanese Patent Application No. 2013-168869 on Aug.15, 2013 including the specification, drawings, and abstract isincorporated herein by reference in its entirety.

BACKGROUND

The present invention concerns a semiconductor device and a method ofmanufacturing the semiconductor device which can be utilized suitably tosemiconductor devices, for example, using nitride semiconductors.

In recent years, semiconductor devices using group III-V compoundshaving a larger bang gap than Si have been noted. Among them,development has been progressed for semiconductor devices which arepower MISFET (Metal Insulator Semiconductor Field Effect Transistor)using gallium nitride and which enable normally off operation in view ofhigh voltage withstanding and high speed switching characteristics.

For example, Japanese Unexamined Patent Application Publication No.2008-306083 discloses a group III-V nitride semiconductor field effecttransistor having a gradient portion at the bottom of a gate electrode.

Further, Japanese Unexamined Patent Application Publication No.2012-248636 discloses a field effect transistor having a channel layerand 2 DEG at a hetero-junction interface between the channel layer andan electron supply layer.

SUMMARY

The present inventors have been engaged in research and development ofsemiconductor devices using the nitride semiconductors as describedabove and have now been under earnest study for the improvement ofcharacteristics of normally off semiconductor devices. In the course ofthe study, it has been found that there is a room for furtherimprovement in the characteristics of the semiconductor devices usingthe nitride semiconductors.

Other subjects and novel features of the invention will become apparentby reading the description of the present specification in conjunctionwith appended drawings.

An outline of typical embodiments disclosed in the present applicationis to be described simply.

A semiconductor device shown in a preferred embodiment disclosed in thepresent application has a gate electrode disposed by way of a gateinsulation film in a trench that extends through a second nitridesemiconductor layer as far as a midway of a first nitride semiconductorlayer. Then, the end of the bottom of the trench has a rounded shape ora chamfered shape.

A method of manufacturing a semiconductor device shown in one embodimentdisclosed in the present application has a step of forming a trench byetching a stack of a first nitride semiconductor layer and a secondnitride semiconductor layer thereover. This is a step of forming atrench that passes through the second nitride semiconductor layer as faras a midway of the first nitride semiconductor layer in which the end ofthe bottom has a rounded shape or a chamfered shape.

According to the semiconductor device shown in the following typicalembodiments disclosed in the present invention can improvecharacteristics of the semiconductor device.

According to the method of manufacturing a semiconductor device shown inthe following typical embodiments disclosed in the present invention, asemiconductor device of preferred characteristics can be manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view illustrating a configuration of asemiconductor device of a first embodiment;

FIG. 2 is a cross sectional view illustrating a step of manufacturingthe semiconductor device of the first embodiment;

FIG. 3 is a cross sectional view illustrating a step of manufacturingthe semiconductor device of the first embodiment, which is a crosssectional view illustrating a manufacturing step succeeding to FIG. 2;

FIG. 4 is a cross sectional view illustrating a step of manufacturingthe semiconductor device of the first embodiment, which is a crosssectional view illustrating a manufacturing step succeeding to FIG. 3;

FIG. 5 is a cross sectional view illustrating a step of manufacturingthe semiconductor device of the first embodiment, which is a crosssectional view illustrating a manufacturing step succeeding to FIG. 4;

FIG. 6 is a cross sectional view illustrating a step of manufacturingthe semiconductor device of the first embodiment, which is a crosssectional view illustrating a manufacturing step succeeding to FIG. 5;

FIG. 7 is a cross sectional view illustrating a step of manufacturingthe semiconductor device of the first embodiment, which is a crosssectional view illustrating a manufacturing step succeeding to FIG. 6;

FIG. 8 is a cross sectional view illustrating a step of manufacturingthe semiconductor device of the first embodiment, which is a crosssectional view illustrating a manufacturing step succeeding to FIG. 7;

FIG. 9 is a plan view illustrating a configurational example of thesemiconductor device of the first embodiment;

FIG. 10 is a cross sectional view illustrating a configuration near atrench of the semiconductor device of the first embodiment;

FIG. 11 is a cross sectional view illustrating a configuration near atrench of a semiconductor device of a first comparative example;

FIG. 12 is a cross sectional view illustrating a configuration near atrench of a semiconductor device of a second comparative example;

FIG. 13 is a cross sectional view illustrating a configuration near atrench of a semiconductor device of a third comparative example;

FIG. 14A is a cross sectional view schematically illustrating aconfiguration of a first modification of the semiconductor device of thefirst embodiment;

FIG. 14B is a cross sectional view schematically illustrating aconfiguration of a second modification of the semiconductor device ofthe first embodiment;

FIG. 15 is a cross sectional view schematically illustrating aconfiguration of a semiconductor device of a second embodiment;

FIG. 16 is a cross sectional view illustrating other configuration forthe end of the bottom of a trench of the semiconductor device of thesecond embodiment;

FIG. 17 is a cross sectional view schematically illustrating aconfiguration of a first example of a semiconductor device of a thirdembodiment;

FIG. 18 is a cross sectional view schematically illustrating aconfiguration of a second example of the semiconductor device of thethird embodiment;

FIG. 19 is a cross sectional view schematically illustrating aconfiguration of a first example of a semiconductor device of a fourthembodiment; and

FIG. 20 is a cross sectional view schematically illustrating aconfiguration of a second example of the semiconductor device of thefourth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following embodiments, the embodiments may be described in aplurality of divided sections or embodiments if required for the sake ofconvenience. However, unless otherwise specified, they are notindependent of each other, but are in such a relation that one is amodification example, an application example, detailed explanation,complementary explanation, or the like of a part or the entirety of theother. Further, in the following embodiments, when reference is made tothe number of elements or the like (including number of piece, numericalvalue, quantity, range, or the like), the number of elements is notlimited to the specified number, but may be greater than or less thanthe specified number unless otherwise specified and except the casewhere the number is apparently limited to the specified number inprinciple.

Further, in the following embodiments, the constitutional elements(including element steps or the like) are not always essential unlessotherwise specified and except the case where they are apparentlyconsidered essential in principle. Similarly, in the followingembodiments, when reference is made to shapes, positional relationships,or the like of the constitutional elements or the like, they includeones substantially analogous or similar to the shapes or the like unlessotherwise specified and expect the case where it is considered that theyare apparently not so in principle. This is also applicable to theforegoing number or the like (including number of piece, numericalvalue, quantity, range, or the like).

Embodiments of the present invention are to be described below indetails by reference to the drawings. Throughout the drawings fordescribing the embodiments, members having the same function are givenwith same or corresponding reference signs, and duplicate descriptiontherefor is omitted. Further, where a plurality of similar members(portions) are present, individual or specified portions are sometimesshown by adding symbols to collective signs. Further, in the followingembodiments, a description for same or similar portions will not berepeated in principle unless it is particularly required.

Further, in the drawings used for embodiments, hatching may sometimes beomitted even in a cross-sectional view for easy understanding of thedrawings. On the other hand, hatching may be sometimes added even in aplan view for easy understanding of the drawings.

Further, in cross sectional views and plan views, the size for each ofportions does not correspond to that of an actual device but a specifiedportion is sometimes depicted relatively larger for easy understandingof the drawings. Further also in the case where a cross sectional viewand a plan view correspond to each other, a specified portion issometimes depicted relatively larger for easy understanding of thedrawing.

First Embodiment

A semiconductor device of this embodiment is to be describedspecifically with reference to the drawings. FIG. 1 is a cross sectionalview illustrating a configuration of a semiconductor device of thisembodiment. FIG. 2 to FIG. 8 are cross sectional views illustratingsteps of manufacturing the semiconductor device of this embodiment.

(Description of Structure)

FIG. 1 is a cross sectional view illustrating a configuration of asemiconductor device according to this embodiment. The semiconductordevice illustrated in FIG. 1 is a MIS (Metal Insulator Semiconductor)field effect transistor (FET) using nitride semiconductors. Thesemiconductor device is also referred to as a high electron mobilitytransistor (HEMT) or a power transistor. The semiconductor deviceaccording to this embodiment is a so-called recessed gate typesemiconductor device.

The semiconductor device of this embodiment has, a stack (nitridesemiconductor region) comprising a nuclear generation layer NUC, astress moderation layer STR, a buffer layer BU, a channel layer (alsoreferred to as electron running layer) CH, and a barrier layer BA formedsuccessively over a substrate S.

Further, a gate GE of the semiconductor device of this embodiment isformed by way of a gate insulation film GI inside a trench T which isengraved so as to pass through an insulation film IF and a barrier layerBA as far as a midway of the channel layer CH.

Further, a source electrode SE and a drain electrode DE of thesemiconductor device of this embodiment are formed on both sides of thegate electrode GE over the barrier layer BA.

Description is to be made specifically. As illustrated in FIG. 1, in thesemiconductor device of this embodiment, the nuclear generation layerNUC is formed over the substrate S, and the stress moderation layer STRis formed over the nuclear generation layer NUC. The nuclear generationlayer NUC is formed in order to form crystal nuclei upon growing oflayers formed above such as the stress moderation layer STR. Further,the nuclear generation layer NUC is formed in order to prevent diffusionof constituent elements in the layers formed above (for example, GA)from the layers formed above to the substrate S, thereby deterioratingthe substrate S. Further, the stress moderation layer STR is formed inorder to moderate stress to the substrate S thereby suppressinggeneration of warps or cracks in the substrate S.

A buffer layer BU is formed over the stress moderation layer STR, achannel layer comprising a nitride semiconductor (also referred to as aelectron running layer) CH is formed over the buffer layer BU, and abarrier layer BA comprising a nitride semiconductor is formed over thechannel layer CH. That is, the buffer layer BU, the channel layer CH,and the barrier layer BA are formed (stacked) from below successivelyover the main surface (upper surface) of the stress moderation layerSTR. A source electrode SE and a drain electrode DE are formedrespectively by way of ohmic layers OL over the barrier layer BA. Thebuffer layer BU is an intermediate layer situated between the channellayer CH and the stress moderation layer STR.

The gate electrode GE is formed by way of a gate insulation film GI tothe inside of a trench (also referred to as trench, recess, concaveportion) that is engraved through the insulation film IF and the barrierlayer BA as far as a midway of the channel layer CH. The sourceelectrode SE and the drain electrode DE are formed on both sides of thegate electrode GE over the barrier layer BA. The source electrode SE andthe drain electrode DE are formed so as to be connected respectivelywith the barrier layer BA. Each of the electrodes is connected by way ofthe ohmic layer OL to provide an ohmic contact.

An insulation layer IL1 is formed over the gate electrode GE. Further,the source electrode SE and the drain electrode DE are formed in theinside and above contact holes C1 formed in the insulation layer IL1. Aninsulation layer IL2 is formed over the insulation layer IL1, the sourceelectrode SE, and the drain electrode DE.

In this embodiment, a two-dimensional electron gas 2 DEG is formed nearthe interface between the channel layer CH and the barrier layer BA onthe side of the channel layer. Further, when a positive potential(threshold potential) is applied to the gate electrode GE, a channel Cis formed near the interface between the gate insulation film GI and thechannel layer CH.

The two-dimensional electron gas 2 DEG is formed by the followingmechanism. Nitride semiconductors constituting the channel layer CH andthe barrier layer BA (gallium nitride type semiconductor in thisembodiment) are different in view of band gap and electron affinityrespectively. Therefore, a well-type potential is formed at the bondedsurface of the semiconductors. When electrons are accumulated in thewell-type potential, the two-dimensional electron gas 2 DEG is formednear the interface between the channel layer CH and barrier layer BA.

The two-dimensional electron gas 2 DEG formed near the interface betweenthe channel layer CH and the barrier layer BA is disconnected by thetrench T in which the gate electrode GE is formed. Accordingly, thesemiconductor device of this embodiment can maintain the off state whena positive potential (threshold potential) is not applied to the gateelectrode GE and can maintain the on state when the positive potential(threshold potential) to the gate electrode GE. As described above, thesemiconductor device can perform normally off operation.

In this embodiment, the end of the bottom of the trench T is rounded. Inother words, the end of the bottom of the trench T has a rounded shape.Thus, when the end of the bottom of the trench T is rounded, thethickness of the gate insulation film GI situated between the end of thebottom of the gate electrode GE and the end of the bottom of the trenchT can be decreased. In other words, the thickness of the gate insulationfilm GI in contact with the end of the bottom of the trench T can bedecreased. This can form a channel C also at the end of the bottom ofthe trench T to reduce the resistance of the channel C. Further, sincethe channel C is formed also at the end of the bottom of the trench Tand, accordingly, the distance between the channel C and thetwo-dimensional electron gas 2 DEG can be decreased, parasiticresistance between them can be reduced.

(Description of Manufacturing Method)

Then, with reference to FIG. 2 to FIG. 8, a method of manufacturing thesemiconductor device of this embodiment is to be described and theconfiguration of the semiconductor device is made clearer. FIG. 2 toFIG. 8 are cross sectional views illustrating the manufacturing steps ofthe semiconductor device of this embodiment.

As illustrated in FIG. 2, a nuclear generation layer NUC, stressmoderation layer STR, and a buffer layer BU are formed successively overa substrate S. A semiconductor substrate, for example, comprisingsilicon Si and exposed at a (111) face is used as the substrate S and,for example, an aluminum nitride (AlN) layer is heteroepitaxially grownas a nuclear generation layer NUC by using an MOCVD (metal organicchemical vapor deposition) method thereover. Then, a super latticestructure formed by repeatedly stacking stacked films each comprising agallium nitride (GaN) layer and an aluminum nitride (AlN) layer (AlN/GaNfilm) is formed as a stress moderation layer STR. For example, thegallium nitride (GaN) layer and the aluminum nitride (AlN) layer areheteroepitaxially grown each at a film thickness of about 2 to 3 nm andeach by 100 layers (200 layers in total) repeatedly by using, forexample, a metal organic chemical vapor deposition method. As thesubstrate S, a substrate comprising SiC, sapphire, or the like may alsobe used in addition to silicon.

Then, a buffer layer BU is formed over the stress moderation layer STR.For example, an AlGaN layer is heteroepitaxially grown as the bufferlayer BU over the stress removing layer STR by using, for example, ametal organic chemical vapor deposition method.

Then, as illustrated in FIG. 3, a channel layer CH is formed over thebuffer layer BU. For example, a gallium nitride (GaN) layer isheteroepitaxially grown over the buffer layer BU by using, for example,a metal organic chemical vapor deposition method.

Then, for example, an AlGaN layer is heteroepitaxially grown as abarrier layer BA over the channel layer CH by using, for example, ametal organic chemical vapor deposition method. The barrier layer BAcomprises a nitride semiconductor having a band gap larger than thechannel layer CH.

As described above, a stack of the buffer layer BU, the channel layerCH, and the barrier layer BA is formed. The stack is formed by theheteroepitaxial growing, that is, by group III growing face stacking in[0001] crystal orientation (C axis). In other words, the stack is formedby the (0001) Ga plane growing. A two-dimensional electron gas (2 DEG)is formed in the stack near the interface between the channel layer CHand the barrier layer BA.

Then, as illustrated in FIG. 4, an insulation film IF having an openingis formed over the barrier layer BA. For example, a silicon nitride filmis deposited as the insulation film IF over the barrier layer BA usingthermal CVD (Chemical Vapor Deposition) or the like. Then, an opening isformed in the insulation film IF by using a photolithographic techniqueand an etching technique.

Then, as illustrated in FIG. 5, the barrier layer BA and the channellayer CH are etched by using the insulation film IF as a mask, therebyforming a trench T that passes through the insulation film IF and thebarrier film BA as far as a midway of the channel layer CH.

In this step, the end of the bottom of the trench T is rounded bycontrolling the etching condition. In other words, the end of the bottomof the trench T has a roundness. As an etching gas, for example, achlorine type gas (for example, BCl₃) is used.

For example, after etching the barrier layer BA and the channel layer CHin a state where the content of an anisotropic etching ingredient islarger and then etching is applied while changing to a state where thecontent of an isotropic etching ingredient is larger than theanisotropic etching ingredient using the insulation film IF as a mask.Thus, the end of the bottom of the trench T can be rounded. Forincreasing the isotropic etching ingredient, etching is applied, forexample, by lowering a bias potential to the substrate S. The end of thebottom of the trench T can be rounded by controlling various etchingconditions, for example, by changing etching gas species, as well asincreasing a plasma density, increasing a gas pressure, and increasing aflow rate of the process gas.

According to the study of the present inventors, a groove T in which theend of the bottom is rounded at a desired trench depth could be formedby applying etching using BCl₃ while controlling the etching condition.

After the etching, a heat treatment (annealing) may also be applied forthe recovery of etching damages.

Then, as illustrated in FIG. 6, a gate insulation film GI is formed overthe insulation film IF including the inside of the trench T. Forexample, aluimina (aluminum oxide film: Al₂O₃) is deposited as a gateinsulation film GI over the insulation film IF including the inside ofthe trench T by CVD or the like. When the deposition method such as CVDis used, the film material is deposited isotropically. As the gateinsulation film GI, a silicon oxide film or a high dielectric filmhaving a dielectric constant higher than that of the silicon oxide filmmay also be used in addition to alumina. As the high dielectric film, ahafnium oxide film (HfO₂ film) may also be used. As the high dielectricfilm, other hafnium type insulation films such as hafnium aluminatefilm, a HfON film (hafnium oxynitride film), a HfSiO film (hafniumsilicate film), a HfSiON film (hafnium silicon oxynitride film), andHfAlO film may also be used.

Then, a gate electrode GE is formed over the gate insulation film GI inthe inside of the trench T. For example, a stack film comprising, forexample, a nickel (Ni) film and a gold (Au) thereon film (also referredto as Au/Ni film) is deposited as a conductive film over the gateinsulation film GI about at a thickness to filling the inside of thetrench T by using sputtering or the like. Then, the Au/Ni film ispatterned by using a photolithographic technique and an etchingtechnique to form a gate electrode GE. Upon etching the Au/Ni film, thegate insulation film GI and the insulation film IF in the lower layermay be etched. Further, as the material for forming the gate electrodeGE, other metal film than the Au/Ni film may also be used and, further,an impurity-containing recrystal silicon film or the like may also beused.

Then, as illustrated in FIG. 7, an insulation layer IL1 is formed overthe gate electrode GE. As the insulation layer IL1, for example, asilicon oxide film is formed over the gate electrode GE and the barrierlayer BA by CVD or the like. Subsequently, contact holes C1 are formedin the insulation layer IL1 by a photolithographic technique and anetching technique. The contact holes C1 are disposed on both sides ofthe gate electrode GE over the barrier layer BA.

Then, as illustrated in FIG. 8, an ohmic layer OL is formed over theinsulation film IL1 including the inside of the contact hole C1. Forexample, a stack film comprising, for example, a titanium (Ti) film andan aluminum (Al) film thereon (also referred to as Al/Ti film) isdeposited over the insulation layer IL1 including the inside of thecontact hole C1 by vapor deposition or the like. Further, a stack filmcomprising, for example, a titanium (Ti) film and a titanium nitride(TiN) film present thereon (also referred to as TiN/Ti film) isdeposited over the Al/Ti film by sputtering or the like. Thus, a stackfilm of the titanium (Ti) film, the aluminum (Al) film, the titanium(Ti) film, and the titanium nitride (TiN) film (also referred to asTiN/Ti/Al/Ti film) is formed and a heat treatment at 550° C. for about30 minutes is applied. By the heat treatment, ohmic contact is formed atthe interface between the TiN/Ti/Al/Ti film and the GaN typesemiconductor. Then, an aluminum alloy film is deposited over theTiN/Ti/Al/Ti film (ohmic layer OL) by sputtering or the like. As thealuminum alloy, an alloy of Al and Si (Al—Si), an alloy of Al—Cu(copper), an alloy of Al, SiN, and Cu (Al—Si—Cu), etc. can be used.Then, the TiN/Ti/Al/Ti film and the aluminum alloy film are patterned byusing a photolithographic technique and an etching technique to form asource electrode SE and a drain electrode DE by way of the ohmic layerOL in the contact holes C1.

Then, an insulation layer (also referred to as a cover film or a surfaceprotection film) IL2 is formed over the insulation layer IL1 includingthe source electrode SE and the drain electrode DE. For example, asilicon oxynitride (SiON) film is deposited as the insulation layer IL2over the insulation layer Ill including the source electrode SE and thedrain electrode DE by CVD or the like (referred to FIG. 1).

By the steps described above, the semiconductor device illustrated inFIG. 1 can be formed. A planar shape (layout) of the source electrodeSE, the drain electrode DE, and the gate electrode GE that configure thesemiconductor device is not restricted particularly and, for example, itmay be a planar shape as illustrated in FIG. 9. FIG. 9 is a plan viewillustrating a configurational example of the semiconductor device ofthis embodiment. In FIG. 9, a region between the source electrode SE andthe gate electrode GE and a region between the gate electrode GE and thedrain electrode DE are formed in the same manner for the sake ofsimplicity.

As illustrated in FIG. 9, source electrodes SE extending in a directionY are disposed each at a predetermined pitch in a direction X. Further,drain electrodes DE extending in a direction Y are disposed each at apredetermined pitch in the direction X. Then, each of the plurality ofthe source electrodes SE and each of the plurality of the drainelectrodes DE are disposed alternately to each other in the direction X.Further, the gate electrode GE extending in the direction Y is disposedbetween each of the plurality of the source electrodes SE and each ofthe plurality of the drain electrodes DE.

The plurality of the drain electrodes DE are connected by a drain padDP. The drain pad DP is disposed so as to extend in the direction X onone side of the drain electrodes DE (upper side in FIG. 9). In otherwords, the plurality of the drain electrodes DE are disposed so as toprotrude in the direction of the axis Y from the drain pad DP extendingin the direction X. Such a configuration is sometimes referred to as acomb-shape.

The plurality of the source electrodes SE are connected by a source padSP. The source pad SP is disposed so as to extend in the direction X onthe other side of the source electrodes SE (lower side in FIG. 9). Inother words, the plurality of the source electrodes SE are disposed soas to protrude in the direction of axis Y from the source pad SPextending in the direction X. Such a configuration is sometimes referredto as a comb-shape.

Gate pads GP are disposed on both sides (right side and left side inFIG. 9) of the source pad SP (drain pad DP). Then, the plurality of thegate electrodes GE are connected electrically with gate interconnects GLdisposed so as to extend in the direction X on the side of the other endof the source electrode SE (lower side in FIG. 9). Further, the gateinterconnects GL extending in the direction of the axis X are connectedelectrically with the gate pads GP disposed on the left end and theright end of the drawing.

As has been described above specifically, according to this embodiment,since the end of the bottom of the trench T is rounded (refer to FIG.1), the thickness of the gate insulation film GI situated between theend of the bottom of the gate electrode GE and the end of the bottom ofthe trench T can be decreased. The effect is to be described withreference to FIG. 10 to FIG. 13. FIG. 10 is a cross sectional viewillustrating the configuration near the trench of the semiconductordevice of this embodiment. FIG. 11 to FIG. 13 are, respectively, crosssectional views illustrating the configurations near the trench of thesemiconductor devices of first to third comparative examples.

In this embodiment, since the end of the bottom of the trench T isrounded as illustrated in FIG. 10, the thickness of the gate insulationfilm GI situated between the end of the bottom of the gate electrode GEand the end of the bottom of the trench T can be decreased. For example,in a first comparative example illustrated in FIG. 11, the end of thebottom of the trench T has a L-shape (corner shape) in the crosssectional view. In such a case, when a gate insulation film GI having asubstantially identical thickness Th is formed on the lateral wall andon the bottom of the trench T, the distance between the end of thebottom of the gate electrode GE and the end of the bottom of the trenchT, that is, the thickness of the gate insulation film GI at the portionis √2×Th (≧Th). √2×Th is a product of square root of 2 and Th. This is“(2)^(1/2)×Th” in other expression.

As described above, the thickness of the gate insulation film GIincreases more at the end of the bottom of the trench T than that at thecentral portion of the bottom of the trench T and on the side wallportion of the trench T. Accordingly, the channel C is less formed atthe end of the bottom of the trench T to increase the resistance of thechannel C. Further, a parasitic resistance is generated at the joinedportion between the channel C and the two-dimensional electron gas 2DEG. As a result, the on resistance of the semiconductor device isincreased.

On the contrary, according to this embodiment (FIG. 10), since the endof the bottom of the groove T is rounded, the thickness of the gateinsulation film GI situated between the end of the bottom of the gateelectrode GE and the end of the bottom of the trench T can be decreased.Thus, the channel C can be formed also at the end of the bottom of thetrench T to reduce the resistance of the channel C. Further, since thechannel C is formed also at the end of the bottom of the trench T and,accordingly, the distance between the channel C and the two-dimensionalelectron gas 2 DEG can be decreased, the parasitic resistance betweenthem can be reduced. As a result, the on resistance of the semiconductordevice can be reduced.

Further, since the end of the bottom of the trench T is a portion wherethe material of the gate insulation film GI deposited successively tothe side wall portion of the trench T and the material for the gateinsulation film GI deposited successively to the bottom of the trench Tare joined, the thickness of the gate insulation film GI tends toincrease. Accordingly, in the L-shaped end of the bottom of the trench Tas in the second comparative example illustrated in FIG. 12, a gateinsulation film GI of a thickness √2×Th or larger may possibly be formed(refer to an arrow in FIG. 12). In the second comparative exampledescribed above, the channel C is less formed and the on resistanceincreases further compared with the case of FIG. 11.

On the contrary, in this embodiment (FIG. 10), since the end of thebottom of the trench T is rounded, the thickness of the gate insulationfilm GI less increases in the portion and the on resistance can bereduced more than in the case of the second comparative exampleillustrated in FIG. 12.

Further, as in a further comparative example illustrated in FIG. 13, asub-trench may sometimes be formed to the end of the bottom of thetrench upon forming the trench T. In such a case, the thickness of thegate insulation film in the sub-trench is also added and the channel Cis less formed at the portion (portion surrounded by a dotted circle inFIG. 13) more and more. Such sub-trench tends to be formed upon etchingof the trench T. On the contrary, in this embodiment (FIG. 10), sincethe etching condition is controlled such that the end of the bottom ofthe trench T can be rounded, the sub-trench described above is lessformed and the on resistance can be reduced further than in the case ofthe third comparative example illustrated in FIG. 13.

As have been described above specifically according to the semiconductordevice of this embodiment, the on resistance of the semiconductor devicecan be reduced effectively.

The manufacturing steps described above are only an example and thesemiconductor device of this embodiment may also be manufactured byother steps than those described above.

Modification

Modifications of this embodiment are to be described. In the embodimentdescribed above, the insulation film IF, the gate insulation film GI,and the gate electrode GE over the barrier layer BA including the insideof the trench T were etched simultaneously. That is, they were patternedeach in an identical planar shape by using an identical resist mask butthey may be formed also to different planar shapes. FIGS. 14A and 14Bare cross sectional views schematically illustrating the configurationof modifications of the semiconductor device of this embodiment. FIG.14A is a cross sectional view of a first modification of a semiconductordevice and FIG. 14B is a cross sectional view of a second modificationof the semiconductor device. In the modifications, since otherconfigurations than those for the insulation film IF, the gateinsulation film GI, and the gate electrode GE, and manufacturing stepsare identical with those of the embodiment described above, descriptiontherefor is to be omitted.

For example, as illustrated in FIG. 14A, after etching a gate insulationfilm GI and a gate electrode GE among an insulation film IF, the gateinsulation film GI, and a gate electrode GE over a barrier layer BAincluding the inside of the trench T, the insulation film IF over thebarrier layer BA may be etched in other step. Etching is applied to theinsulation film IF, for example, before the step of forming a sourceelectrode SE and a drain electrode DE. Further, the insulation film IFmay be etched upon forming contact holes C1 after forming an insulationlayer IL1 over the insulation film IF (refer to FIG. 7).

In this case, as illustrated in FIG. 14A, the gate electrode GE isformed by way of a gate insulation film GI in the inside of a trench Twhich is engraved passing through an insulation film IF and a barrierlayer BA as far as a midway of a channel layer CH. A source electrode SEand a drain electrode DE are formed on both sides of the gate electrodeGE over the barrier layer BA. Then, the insulation film IF below thegate insulation film GI is disposed so as to extend from the end of thegate insulation film GI and the gate electrode GE to the sourceelectrode SE. Further, the insulation film IF below the gate insulationfilm GI is disposed so as to extend from the end of the gate insulationfilm GI and the gate electrode GE to the drain electrode DE.

Further, as illustrated in FIG. 14B, each of an insulation film IF, agate insulation film GI, and a gate electrode GE over a barrier layer BAincluding the inside of the trench T may be formed to different planarshapes. In this case, among the insulation film IF, the gate insulationfilm GI, and the gate electrode GE over the barrier layer BA includingthe inside of the trench T, the gate insulation film GI is etched afteretching the gate electrode GE. In this case, the gate insulation film GIbelow the gate electrode GE is disposed so as to extend from the end ofthe gate electrode GE to the source electrode SE or to the drainelectrode DE. Then, after etching the gate insulation film GI, theinsulation film IF over the barrier layer BA is etched. The insulationfilm IF is etched, for example, before the step of forming the sourceelectrode SE and the drain electrode DE. Further, the insulation film IFmay be etched also after forming an insulation layer IL1 over theinsulation film IF and forming the contact holes CI (refer to FIG. 7).

Also in this case, as illustrated in FIG. 14B, the gate electrode GE isformed by way of the gate insulation film GI in the inside of the trenchT which is engraved passing through the insulation film IF and thebarrier layer BA as far as a midway of the channel layer CH. The sourceelectrode SE and the drain electrode DE are formed on both sides of thegate electrode GE over the barrier layer BA. The gate insulation film GIbelow the gate electrode GE is disposed so as to extend from the end ofthe gate electrode GE to the source electrode SE. Further, the gateinsulation film GI below the gate electrode GE is disposed so as toextend from the end of the gate electrode GE to the drain electrode DE.Further, the insulation film IF below the gate insulation film GI isdisposed so as to extend from the end of the gate insulation film GI andthe gate electrode GE to the source electrode SE. Further, theinsulation film IF below the gate insulation film GI is disposed so asto extend from the end of the gate insulation film GI and the gateelectrode GE to the drain electrode DE.

Second Embodiment

In the first embodiment, the end of the bottom of the trench T wasrounded, that is, the end of the bottom of the trench T had a roundness.However, it is not always necessary that the shape for the end of thebottom of the trench T is a curved shape but it may also be a shape inwhich a L-shaped end of the bottom of the trench T is chamfered. FIG. 15is a cross sectional view schematically illustrating a configuration ofa semiconductor device of this embodiment. In this embodiment, sinceother configurations than those of the trench T and the manufacturingsteps are identical with those of the first embodiment, descriptiontherefor is to be omitted.

As illustrated in FIG. 15, in the semiconductor device of thisembodiment, the end of the bottom of the trench T has two-step tapers(tapered portion) TP1 and TP2.

The end of the bottom of the trench T has two-step tapers TP1 and TP2comprising the TP1 having a normal vector at an angle of 22.5° relativeto the surface of the channel layer CH or the barrier layer BA orrelative to the (0001) Ga plane as the surface of the central portion ofthe bottom of the trench T, and the taper TP2 having a normal vector atan angle of 67.5° to the surface described above. The tapers TP1 and TP2are formed continuously. Further, the taper TP1 is disposed continuouslyfrom the side wall of the trench T. The TP2 is disposed below the taperTP1 and disposed continuously from the bottom of the trench T. Further,an angle θ2 formed between the taper TP2 and the bottom of the trench Tis smaller than the angle θ1 formed between the taper TP1 and the bottomof the trench T (θ1>θ2).

By providing the two-step taper structure (TP1, TP2) to the end of thebottom of the trench T, the distance between the end of the bottom ofthe gate electrode GE and the end of the bottom of the trench T, thatis, the thickness of the gate insulation film GI at the portion can beequal to or smaller than Th. Therefore, according to the configurationof this embodiment (FIG. 15), the thickness of the gate insulation filmGI situated between the end of the bottom of the gate electrode GE andthe end of the bottom of the trench T (average thickness) can bedecreased to less than the case of the first embodiment (refer to FIG.10, etc.). “Thickness (average film thickness)” referred to herein isdefined, for example, as a thickness of the gate insulation film GI(average film thickness) in a region which is the end of the bottom ofthe trench T and determined by an extension line along the surface ofthe gate insulation film GI on the lateral side of the trench T and anextension line along the surface of the gate insulation film GI at thebottom of the trench T (refer to a fragmentary enlarged view in FIG.15).

By decreasing the thickness of the gate insulation film GI (average filmthickness), a channel C can be formed also at the end of the bottom ofthe trench T to reduce the resistance of the channel C. Further, sincethe channel C is formed also at the end of the bottom of the trench Tand, accordingly, the distance between the channel C and thetwo-dimensional electron gas 2 DEG can be decreased, a parasiticresistance between them can be reduced. As a result, the on resistanceof the semiconductor device can be reduced.

In FIG. 15, the two-step tapers TP1 and TP2 are formed at the end of thebottom of the trench T, but tapers of three or more steps may also beprovided. Also in this case, an angle formed by a taper situated inlower layer decreases successively. For example, an angle θn formed by anth taper TPn and an angle θn+1 formed by a n+1th taper TPn+1 situatedtherebelow are in a relation of: θn>θn+1.

Further, a single taper may be provided at the end of the bottom of thetrench T. FIG. 16 is a cross sectional view illustrating otherconfiguration for the end of the bottom of the trench of thesemiconductor device in this embodiment.

In the semiconductor device illustrated in FIG. 16, the end of thebottom of the trench T has a taper TP. For example, a taper TP having anormal vector at an angle of 45° is provided relative to the surface ofa channel layer CH or a barrier layer BA, or to the (0001) Ga planewhich is the surface of the central portion of the bottom of the trenchT at the and of the bottom of the trench T. The taper TP is disposedcontinuously from the lateral wall of the trench T, and the taper TP isdisposed continuously from the bottom of the trench T.

In this case, the distance between the end of the bottom of the gateelectrode GE and the end of the bottom of the trench T, that is, thethickness of the gate insulation film GI at the portion can be √2×Th/2.√2×Th/2 is one-half of a product of square 2 and Th, that is,“(2)^(1/2)×Th÷2” in other expression. Accordingly, in the configurationillustrated in FIG. 16, the thickness of the gate insulation film GIsituated between the end of the bottom of the gate electrode GE and theend of the bottom of the trench T can be decreased further. Thus, thechannel C is formed also at the end of the bottom of the trench T andthe resistance of the channel C can be reduced. Further, since thechannel C is formed also at the end of the bottom of the trench T and,accordingly, the distance between the channel C and the two-dimensionalelectron gas 2 DEG can be decreased, a parasitic resistance between themcan be reduced. As a result, the on resistance of the semiconductordevice can be reduced.

In the configuration of this embodiment, the angle and the length of thenormal vector of each taper has been explained geometrically for thesake of simplicity of the explanation but the angle and the length ofthe normal vector of each taper are not restricted to them. That is, thethickness of the gate insulation film GI (average film thickness) at theend of the bottom of the trench T can be √2×Th/2 or larger and smallerthan √2×Th by forming the end of the bottom of the trench T with onetaper or a plurality of continuous tapers and, ultimately, by roundingthe end of the bottom as illustrated in the first embodiment. Thethickness of the gate insulation film GI (average film thickness) at theend of the bottom of the trench T is more preferably within a range ofTh or larger and √2×Th×0.8 or smaller. By decreasing the thickness ofthe gate insulation film GI (average film thickness) at the end of thebottom of the trench T, the resistance of the channel C can be reducedto lower the on resistance of the semiconductor device.

The two-step tapers (TP1, TP2) or the taper TP can be formed bycontrolling the etching condition when the trench T is formed. The taper(TP1, TP2, TP) can be formed at the end of the bottom of the trench T,for example, by applying etching in a state where the content ofanisotropic etching ingredient is large in the same manner as in thefirst embodiment and then controlling various etching conditions (biascondition, etching gas species, gas flow rate, gas pressure, plasmadensity, etc.), for example, by applying etching in a state where thecontent of isotropic etching ingredient is larger than the anisotropicetching ingredient.

In the first and the second embodiments described above, the crosssectional shape at the end of the bottom of the trench has beenexplained as a simple arcuate shape or as a tapered shape at apredetermined angle but this is merely an example and the shape is notrestricted to the example. That is, so long as the thickness of the gateinsulation film GI for the end at the bottom of the trench T can bedeceased by forming the end at the bottom of the trench as a roundedshape, or forming the end at the bottom of the trench as a chamferedshape, R (radius of curvature) of the roundness or the tapered angle ofthe chamfering can take various values.

Third Embodiment First Example

In the first embodiment, the lateral side of the trench T has beenexplained as substantially perpendicular to the (0001) Ga plane which isthe surface of the central portion of the bottom of the trench T(θ=90°), but the lateral side (lateral wall) of the trench T may also beformed as a tapered shape. FIG. 17 is a cross sectional viewschematically illustrating the configuration of a first example of asemiconductor device in this third embodiment. In this embodiment, sinceother configurations than those for the trench T and the manufacturingsteps are identical with those of the first embodiment, descriptiontherefor is to be omitted.

As illustrated in FIG. 17, an angle (tapered angle θ) formed between thelateral wall of the trench T and the surface of the barrier layer BA orthe channel layer CH or the bottom of the trench T, that is, the (0001)Ga plane is smaller than 90° (θ<90°).

For example, the tapered angle θ can be controlled to about 60 to 80° byusual dry etching using BCl₃.

As described above, by forming the lateral side of the trench T as atapered shape, joining of the material for the gate insulation film GIdeposited successively on the lateral wall of the trench T and thematerial for the gate insulation film GI deposited successively over thebottom of the trench T can be moderated to suppress increase of thethickness of the gate insulation film GI.

Further, when the lateral side of the trench T is formed as the taperedshape, as well as the end of the bottom of the trench T is formed as arounded or chamfered shape, the thickness of the gate insulation film GIsituated between the end of the bottom of the gate electrode GE and theend of the bottom of the trench T can be decreased to reduce the onresistance of the semiconductor device as has been described in thedetails for the first and second embodiments.

Second Example

In the first example, the tapered lateral side is formed to the trench Tbut the lateral side and the bottom of the trench T may be roundedintegrally. FIG. 18 is a cross sectional view schematically illustratingthe configuration of a second example of the semiconductor device of asecond embodiment. In this embodiment, since other configurations thanthose of the trench T and the manufacturing steps are identical withthose of the first embodiment, description therefor is to be omitted.

As illustrated in FIG. 18, in the second example, the lateral wall andthe bottom of the trench T are rounded integrally. In other words, thelateral wall and the bottom of the trench T are integrally rounded.

That is, a trench (concave portion) T having an arcuate cross sectionalshape is formed by etching a barrier layer BA and a channel layer CHusing an insulation film IF having an opening as a mask over the barrierlayer BA. Such a trench (concave portion) T can be formed by controllingthe etching condition (for example, applying mainly anisotropicetching).

As described above, by integrally rounding the lateral surface and thebottom of the trench T, the thickness of the gate insulation film GIdeposited in the inside of the trench can be formed uniformly andincrease of the film thickness can be suppressed.

Further, by integrally rounding the lateral side and the bottom of thetrench T, the on resistance of the semiconductor device can be reducedas has been described for the first and the second embodiments.

Fourth Embodiment First Example

In the first embodiment (FIG. 10), the bottom of the gate electrode GEis shown at a position lower than the position for the surface of thebarrier layer BA, but the bottom of the gate electrode GE may besituated also at a position higher than the position for the surface ofthe barrier layer BA. FIG. 19 is a cross sectional view schematicallyillustrating the configuration of a first example of the semiconductordevice of a fourth embodiment. In this embodiment, since otherconfigurations than those of a trench T, a gate insulation film GI, anda gate electrode GE, and manufacturing steps are identical with those ofthe embodiment described above, description therefor is to be omitted.

In the first example, as illustrated in FIG. 19, the bottom of the gateelectrode GE above the trench T is situated at a position higher by adistance D than the position for the surface of the barrier layer BA.

For example, assuming the depth of the trench T, that is, the distancefrom the surface of the barrier layer BA to the bottom of the trench Tas about 40 nm, and the thickness of the gate insulation film GI asabout 100 nm, the inside of the trench T is filled with the gateinsulation film GI and the distance D between the surface of the gateinsulation film GI (bottom of the gate electrode GE) and the surface ofthe barrier layer BA is about 60 nm.

As described above, when the bottom of the gate electrode GE is situatedat a position higher than the position for the surface of the barrierlayer BA, stress caused by thermal expansion of the gate electrode GEthat exerts on the lateral wall or the bottom of the trench T(particularly, end of the bottom of the trench T) can be moderated. Forexample, stresses caused in the heat treatment step subsequent to thestep of forming the trench T and the gate electrode GE (for example,heat treatment step in the step of forming the ohmic layer OL, thesource electrode SE, and the drain electrode DE, specifically, a heattreatment at about 550° C. for 30 minutes) can be moderated.

Further, when the bottom of the gate electrode GE is situated to aposition higher than the position for the surface of the barrier layerBA, and the end of the bottom of the trench T is formed in a rounded orchamfered shape, the thickness of the gate insulation film GI situatedbetween the end of the bottom of the gate electrode GE and the end ofthe bottom of the trench T can be decreased to reduce the on resistanceof the semiconductor device as has been described specifically in thefirst and the second embodiments.

Second Example

In the third embodiment (FIG. 17), the bottom of the gate electrode GEis shown at a position lower than the position for the surface of thebarrier layer BA but the bottom of the gate electrode GE may be situatedalso at a position higher than the position for the surface of thebarrier layer BA. FIG. 20 is a cross sectional view schematicallyillustrating the configuration of a second example of the semiconductordevice of this fourth embodiment. In this embodiment, since otherconfigurations than those of the trench T, the gate insulation film GI,and the gate electrode GE, and manufacturing steps are identical withthose of the embodiment described above, explanation therefor is to beomitted.

In the second example of this embodiment, as illustrated in FIG. 20, thebottom of the gate electrode GE above the trench T is disposed at aposition higher by a distance D than the position for the surface of thebarrier layer BA.

For example, assuming the depth of the trench T, that is, the distancefrom the surface of the barrier layer BA to the bottom of the trench Tas about 40 nm, the thickness of the gate insulation film GI as about100 nm, and the tapered angle as about 60°, the inside of the trench Tis filled with the gate insulation film GI and the distance D betweenthe surface of the gate insulation film GI (bottom of the gate electrodeGE) and the surface of the barrier layer BA is about 60 nm.

As described above, when the bottom of the gate electrode GE is situatedat a position higher than the position for the surface of the barrierlayer BA, stress caused by thermal expansion of the gate electrode GEthat exerts on the lateral wall or the bottom of the trench T(particularly, end of the bottom of the trench T) can be moderated. Forexample, stresses caused by a heat treatment step subsequent to the stepof forming the trench T and the gate electrode GE (for example, heattreatment step in the step of forming the ohmic layer OL, the sourceelectrode SE, and the drain electrode DE, specifically, a heat treatmentat about 550° C. for 30 minutes) can be moderated.

As described in the third embodiment, by forming the lateral side of thetrench T as the tapered shape, joining of the material for the gateinsulation film GI deposited successively on the lateral wall of thetrench T and the material for the gate insulation film GI depositedsuccessively over the bottom of the trench T can be moderated tosuppress increase of the thickness of the gate insulation film GI.

Further, when the end of the bottom of the trench T is formed as therounded or chamfered shape, the thickness of the gate insulation film GIsituated between the end of the bottom of the gate electrode GE and theend of the bottom of the trench T can be decreased to reduce the onresistance of the semiconductor device as has been describedspecifically in the first to third embodiments.

While the inventions made by the present inventors have been describedspecifically with reference to the preferred embodiments, it will beapparent that the invention is not restricted to the embodiments but canbe modified variously within a range not departing the gist of theinventions.

What is claimed is:
 1. A semiconductor device comprising: a first nitride semiconductor layer formed above a substrate; a second nitride semiconductor layer formed over the first nitride semiconductor layer and having a band gap larger than that of the first nitride semiconductor layer; a trench passing through the second nitride semiconductor layer as far as a midway of the first nitride semiconductor layer; and a gate electrode disposed by way of a gate insulation film in the inside of the trench, wherein the end of the bottom of the trench is in a rounded shape and the gate insulation film in contact with the end of the bottom of the trench is in a rounded shape.
 2. The semiconductor device according to claim 1, wherein the thickness of the gate insulation film situated at the end of the bottom of the trench is √2×Th/2 or larger and smaller than √2×Th assuming the thickness of the gate insulation film on the side wall of the trench as Th.
 3. The semiconductor device according to claim 1, wherein the thickness of the gate insulation film situated to the end of the bottom of the trench is Th or larger and √2×Th×0.8 or smaller assuming the thickness of the gate insulation film on the side wall of the trench as Th.
 4. The semiconductor device according to claim 1, wherein the device has a first electrode and a second electrode formed respectively on both sides of the gate electrode over the second nitride semiconductor layer.
 5. The semiconductor device according to claim 1, wherein the side wall of the groove is in a tapered shape.
 6. The semiconductor device according to claim 1, wherein the trench is a concave portion having an arcuate cross section.
 7. The semiconductor device according to claim 1, wherein the bottom of the gate electrode above the trench is situated at a position higher than the position for the surface of the second nitride semiconductor layer.
 8. A semiconductor device comprising: a first nitride semiconductor layer formed above a substrate; a second nitride semiconductor layer formed over the first nitride semiconductor layer and having a band gap larger than that of the first nitride semiconductor layer; a trench passing through the second nitride semiconductor layer as far as a midway of the first nitride semiconductor layer; and a gate electrode disposed by way of a gate insulation film in the inside of the trench, wherein the end of the bottom of the trench is in a chamfered shape and the gate insulation film in contact with the end of the bottom of the trench is in a chamfered shape.
 9. The semiconductor device according to claim 8, wherein the side wall of the groove is in a tapered shape.
 10. The semiconductor device according to claim 8, wherein the end of the bottom of the trench comprises a first tapered portion and a second tapered portion situated below the first tapered portion, and the angle formed between the second tapered portion and the bottom of the trench is smaller than the angle formed between the first tapered portion and the bottom of the trench.
 11. The semiconductor device according to claim 9, wherein the end of the bottom of the trench comprises a plurality of tapered portions.
 12. The semiconductor device according to claim 9, wherein the thickness of the gate insulation film situated at the end of the bottom of the trench is √2×Th/2 or larger and smaller than √2×Th assuming the thickness of the gate insulation film on the side wall of the trench as Th.
 13. The semiconductor device according to claim 9, wherein the thickness of the gate insulation film situated at the end of the bottom of the trench is Th or larger and √2×Th×0.8 or smaller assuming the thickness of the gate insulation film on the side wall of the trench as Th.
 14. The semiconductor device according to claim 8, wherein the device has a first electrode and a second electrode formed respectively on both sides of the gate electrode over the second nitride semiconductor layer.
 15. The semiconductor device according to claim 8, wherein the side wall of the trench is in a tapered shape.
 16. The semiconductor device according to claim 8, wherein the bottom of the gate electrode above the trench is situated at a position higher than the position for the surface of the second nitride semiconductor layer.
 17. A method of manufacturing a semiconductor device comprising the steps of: (a) forming a first nitride semiconductor layer, and forming a second nitride semiconductor layer having a band gap larger than that of the first nitride semiconductor layer over the first nitride semiconductor layer, thereby forming a stack; (b) etching the stack thereby forming a trench; (c) forming a gate insulation film in the inside of the trench; and (d) forming a gate electrode over the gate insulation film, wherein the step (b) is a step of forming the trench passing through the second nitride semiconductor layer as far as a midway of the first nitride semiconductor layer, and has a rounded or chamfered shape provided at the end of the bottom thereof.
 18. The method of manufacturing the semiconductor device according to claim 17, wherein the step (c) is a step of forming the gate insulation film by a chemical vapor deposition method.
 19. The method of manufacturing the semiconductor device according to claim 18, wherein the thickness of the gate insulation film situated at the end of the bottom of the trench is √2×Th/2 or larger and smaller than √2×Th assuming the thickness of the gate insulation film on the side wall of the trench as Th.
 20. The method of manufacturing the semiconductor device according to claim 18, wherein the thickness of the gate insulation film situated at the end of the bottom of the trench is Th or larger and √2×Th×0.8 or smaller assuming the thickness of the gate insulation film on the side wall of the trench as Th. 